What is the "the dos compatibility flag" in fdisk?

Here is your answer. DOS doesn't allow a partition to start (or end) mid-cylinder. It assumes the partition table is corrupt when it sees this, and not only won't boot off of that partition, it won't boot any partition on the disk.

Basically, unless you're dual-booting, turn DOS compatibility off. Even if you are, unless you're running an old version of DOS/Windows, turn it off.